The present invention relates to improvements in methods and apparatus for treating tows of filamentary filter material, especially for converting tows of filamentary filter material into fillers of filter rods which are thereupon subdivided into sections of unit length or multiple unit length for assembly with plain cigarettes, cigars or cigarillos into filter tipped smokers' products.
Commonly owned U.S. Pat. No. 3,971,695 to Block discloses a method of converting a tow of pretreated filamentary filter material into a continuous rod-like filler which is thereupon draped into a web of cigarette paper or other suitable wrapping material to form therewith a continuous filter rod which is ready to be subdivided into sections of desired length preparatory to admission of such sections into a reservoir for curing of the plasticizer which is applied to the filaments or directly into a filter tipping or other processing machine.
At the present time, the conversion of a tow into a rod-like filler normally takes place in a nozzle (also called aspirating jet) through which the tow is caused to advance to be thereby subjected to the action of compressed air with attendant densification ahead of the wrapping station. A suitable nozzle is disclosed in U.S. Pat. No. 3,016,945 to Wexler. It is customary to regulate the mass of filter material in the filler in order to ensure that the density of each of a long series of successively formed filter rod sections will be the same. The weight and density of filter material in the filler are regulated by varying the pressure of compressed air which is supplied to the aspirating jet or nozzle. Such regulation of the pressure of air which is admitted into the nozzle is intended to ensure that the weight of each filter rod section will match a preselected optimum value even if the weight of individual filaments deviates from an optimum weight. In many presently known filter rod making machines, the means for monitoring the weight and hence the density of successive unit lengths of the filler downstream of the nozzle comprises a density measuring device of the type known as NSR which employs a source of corpuscular radiation and is manufactured and sold by the assignee of the present application. It is also possible to employ a suitable weighing device or an X-ray machine. All that counts is to ensure that the density of filter material in successively formed filter rod sections will match, or will not deviate appreciably from, a preselected optimum value.
It has been found that, when a filter rod making machine is operated at less than nominal speed, the resistance of the fillers of filter rod sections to the flow of a gaseous fluid therethrough fluctuates within a very wide range. In other words, the so-called standard deviation or mean square deviation of such resistance is much more pronounced than when the filter rod sections are produced while the machine operates at the rated speed. Such pronounced fluctuations of standard deviation are surprising since the resistance of a filter rod section to the flow of a gaseous fluid therethrough is a function of the density of the filler, i.e., of the mass of filamentary material which fills the tubular wrapper of the filter rod section. The mass is constant irrespective of the speed of the filter rod, i.e., regardless of the selected operating speed of the filter rod making machine.